Conventionally, when power is interchanged with each other in direct-current systems, for example, it is proposed to use a bidirectional DC-DC converter to achieve a high direct-current voltage with a small-scale circuit.
For example, a conventional bidirectional DC-DC converter (JP2002-165448A, for example) has bidirectional direct-current/alternating-current converting parts on the both sides of a transformer.
In a secondary-side direct-current/alternating current converting part of the bidirectional DC-DC converter, a choke coil acting as a smoothing coil is used as a choke coil of a choke coil-using chopper circuit-type inverter in forward power transmission, and a switching and rectifying part between the choke coil and a secondary coil of the transformer functions as a rectifier in forward power transmission. On the other hand, in reverse power transmission, the switching and rectifying part functions as a chopper circuit.
However, with the conventional bidirectional DC-DC converter described in the 3P2002-165448A described above, for example, in a step-down operation, a switching loss occurs because of variations in characteristics between MOS transistors serving as switching elements, and the converter efficiency decreases.
And in hard switching, problems arise that noise increases and breakdown of switching elements occurs.